`UserDict` was moved to the [http://docs.python.org/py3k/library/collections.html#collections.UserDict collections] module in Python 3.x. We can add a twisted.python.compat version, like we did with `execfile` or it will throw errors in python 3.

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Remove usage of `UserDict` once Python 2.6 support is dropped completely (or `manhole.explorer` or `protocols.postfix` are ported to python 3).